1. Field of the Invention
The invention is related to the field of telecommunications, and in particular, to communication systems and methods with phase encoding and optical filtering.
2. Description of the Prior Art
Communication systems use optical signals to communicate over optic fiber. Some of these systems transmit optical signals with a carrier frequency and sidebands. These sidebands carry the user data and are at frequencies slightly above and below the carrier frequency. In Optical Vestigial Sideband (OVSB) transmission, a portion of the redundant sideband is removed to form a vestigial sideband signal because a vestige of the sideband remains. Some prior systems have used optical filters in OVSB transmission. One example of an optical filter is a Fiber Bragg Grating (FBG) filter. Ultra-violet light is shined on an FBG filter to generate gratings that in turn interacts with an optical signal for filtering the optical signal. One problem with optical filters in general is the filtering is non-ideal, where the target frequencies are not completely filtered, and the neighboring non-target frequencies are partially filtered. OVSB does reduce the channel bandwidth, but one problem with OVSB is the residual sideband still occupies a significant portion of the transmission spectrum and limits the spectral efficiency.
In Optical Single Sideband (OSSB) transmission, one of the sidebands is almost completely removed. Some prior systems using OSSB transmission have used electrical sub-carrier techniques. However, these systems require complicated analog circuitry. Also, the data rate is limited. One goal for designing systems that use OSSB transmission is to have a main signal power spectrum with no or negligible DC and low frequency components.
Prior electrical systems have used various encoding techniques for better binary signal performance. The simplest technique is non-return to zero (NRZ), where a binary 1 is represented by optical power within a bit period and a binary 0 is represented by zero optical power. Another technique is return to zero (RZ), where a binary 1 is an optical pulse while binary 0 means no optical power. Other types of encoding use phase transition of the signal to indicate a 1 or a 0, which is called phase encoding. One example of phase encoding is Manchester encoding. In Manchester encoding, a logical 0 is by a transition at the edge of a bit period, while a logical 0 is represented by a transition at the bit center. One problem with Manchester coding optical signals is the signal spectrum is doubled as compared with NRZ line coding.
Another example of phase encoding is Miller encoding, which is also called delay modulation. In Miller encoding, a logical 1 is represented with a phase transition at the bit center. A logical 0 is represented with no phase transition at the bit center. Two consecutive logical 0 has a phase transition at the boundary of the end of the first bit. Miller coding concentrates signal's power spectral density such that the signal's spectral occupancy is narrow. Miller coding has not been used in externally-modulated optical communication systems.